1. Field of the Invention
The present invention relates to a hologram recording/reproducing apparatus for multiplex recording holograms in a certain recording area of a hologram recording medium, as well as a hologram multiplex recording method, and a hologram recording medium.
2. Description of the Background Art
In recent years, as information technology such as information digitization has been rapidly developed, various contents information such as video or audio information are distributed to users at a high speed with high quality for users' utilization. As for the users' utilization, it is often the case that each user temporarily saves the distributed information in e.g. a hard disk, and then selects and/or edits the information which he or she wishes to store for a long term for storage into another recording medium. In view of this, there is a demand for establishing a system which enables to realize an ultra-high-speed recording/reproducing and an ultra-large-capacity storage in order to cope with an explosive increase in information amount concerning high-quality information and diversified demands in information utilization.
As one promising means for meeting the aforementioned demands, there has been proposed a hologram optical information recording/reproducing apparatus (also called as a hologram recording/reproducing apparatus, a hologram memory, or a holographic memory) which utilizes photon mode optical information recording. The hologram recording/reproducing system is a system configured in such a manner that a signal beam modulated depending on data to be recorded, and a reference beam are recorded in a hologram recording medium as an interference fringe, and the data recorded in the hologram recording medium is reproduced by causing a readout beam substantially equivalent to the reference beam to be incident onto the hologram recording medium recorded with the data. Hereinafter, the readout beam is also called as “reference beam for reproduction”. Generally, a reference beam to be used in data recording is used as the reference beam for reproduction.
For instance, there is known a hologram memory optical system according to a shift multiplex recording system proposed by Psaltis et al., as an example of the hologram optical information recording system. In the system proposed by Psaltis et al., after the light emitted from a laser light source has its beam diameter expanded by a beam expander, the light beam is divided by a half mirror. After the beam division, one of the beams is transmitted through a spatial light modulator, condensed on a hologram recording medium by a Fourier transform lens, and turned into a signal beam. The other of the beams is turned into a reference beam for irradiating the same position on the hologram recording medium as the signal beam. The hologram recording medium is produced by e.g. sealably placing a hologram medium such as a photopolymer between e.g. two glass substrates to record an interference fringe by the signal beam and the reference beam.
The spatial light modulator is a two-dimensionally-arranged light switch array such as a liquid crystal panel or a DMD (digital micro mirror device), and is operated in such a manner that the light switches are independently turned on or off in response to an input signal to be recorded. For instance, if a spatial light modulator of 1,024×1,024 cells is used, one megabit information can be simultaneously displayed. When a signal beam is transmitted through the spatial light modulator, one megabit information to be displayed on the spatial light modulator is transformed into a two-dimensional light beam array, and recorded on a hologram recording medium as an interference fringe. In reproducing the recorded signal, merely the reference beam is irradiated onto the hologram recording medium, and a diffraction beam (also, called as “reproduction beam”) from the hologram is received by a two-dimensional image acquirer such as a CMOS sensor or a CCD device.
The aforementioned hologram optical information recording system has the following features. Since the thickness of the hologram medium is as large as about 1 mm, data is recorded as a grating having a large interference fringe i.e. so-called Bragg grating, which allows for multiplex recording, and enables to realize a large-capacity optical recording system. The multiplex recording is one of the primary features of the hologram recording/reproducing. The multiplex recording/reproducing system has also been actively developed. For instance, in the aforementioned shift multiplex recording system, a spherical wave is used in hologram recording. With this arrangement, multiplex recording can be performed in such a manner that a certain area recorded with a hologram, and another area recorded with a succeeding hologram are partly lapped one over the other by shifting the two areas one from the other by a certain amount capable of selectively reproducing the two areas.
There are known, as other examples of the multiplex recording system, an angular multiplexing system, wherein multiplex recording/reproducing is performed with respect to a common area by changing an incident angle of one or both of a reference beam and a signal beam onto a hologram recording medium for each hologram recording; and a peristrophic multiplexing system, wherein multiplex recording is performed by rotating an incident direction of the reference beam or the signal beam onto a recording medium with respect to a normal of the recording medium.
In the angular multiplexing system, the incident angle is changed by mechanical means such as a galvanometric mirror, or electric means such as a deflector using an acoustic optical device or an electro-optic device. As a method for removing crosstalk between holograms to be formed by multiplex recording, there is proposed a polytopic multiplexing system, in which merely a reproduction beam is substantially extracted from a hologram by subjecting adjacent and multiplex-recorded holograms to be reproduced simultaneously to filtering by using an aperture or a like device. Also, there is proposed an approach of constituting a light flux deflector and a deflection controller of a wedge prism and a rotation manipulator for rotating the wedge prism, and performing an angular multiplexing recording and a peristrophic multiplexing recording in combination with each other. Further, there is proposed a system of performing an angular multiplexing recording with use of a spherical reference wave, while changing an incident angle of a reference beam.
The above arrangement utilizes that the incident angle of the reference beam to be received on individual parts of a disk-shaped hologram recording medium is slightly changed when the recording position is shifted by slightly rotating the recording medium. In the case where the thickness of the hologram medium is 1 mm, wavelength selectivity defined by a reproduction signal intensity is 0.014 degree in full width at half maximum. If holograms are multiplex-recorded at a pitch of about 20 μm, with use of 0.5 in numerical aperture (NA) of a reference beam, and 2 mmφ in hologram size, the recording density is 600 Gbit/inch2. Thus, a hologram recording medium of 730 GB in terms of a 12 cm-disk capacity is provided.
Another multiplex recording system is proposed. For instance, with use of means for changing an incident angle or a phase distribution of a reference beam each time two-dimensional data is holographically recorded, it is possible to multiplex record holograms in one recording area of a hologram recording medium. By these multiplex recording systems, data can be recorded with extremely high density, which is advantageous in spectacularly increasing the recording capacity, as compared with a conventional optical disk such as a compact disk (CD) or a digital versatile disk (DVD).
With use of the aforementioned system, two-dimensional data displayed on the spatial light modulator can be concurrently recorded and reproduced. This enables to realize overwhelming high-speed data access, as compared with a CD or a DVD. The hologram recording/reproducing method including the multiplex recording system is primarily proposed to enhance the recording capacity by increasing the multiplicity, and an object thereof is to provide a multiplex recording principle/approach with respect to a hologram recording medium.
Generally, the hologram multiplex recording technique is directed to forming multiple holographic conditions in a common area of a hologram recording medium. Conventionally, the hologram multiplex recording technique has been realized by changing an angle, wavelength, phase code or a like parameter in recording or reproducing holograms. The common area is a volumetric area where at least parts of multiple holograms are lapped one over the other i.e. an area including an in-plane direction and a thickness direction of a recording medium. Many of the multiplexing recording method utilize light diffraction under Bragg conditions in order to separate the holograms one from the other for recording/reproducing.
For instance, in case of the angular multiplexing recording system, multiplex recording is performed by recording an interference fringe pattern to be formed by a signal beam carrying certain two-dimensional data, and a reference beam to be emitted from a light source for emitting the signal beam, as a refractive index distribution. The interference fringe pattern serves as a hologram. A diffraction beam corresponding to an intended hologram is obtained exclusively in a condition that the incident angle of a readout beam substantially coincides with the incident angle of the signal beam or the reference beam to be used in recording a hologram in the recording medium.
According to the multiplex recording system, independently readable holograms can be multiplex-recorded in a common volumetric area by changing the incident angle of at least one of the signal beam and the reference beam onto the recording medium. Now, let it be assumed that a change amount in incident angle capable of separating and reproducing a hologram in the common area is Δθ, and the sum of the change amounts of the incident angles of the signal beam and the reference beam is θ, then, θ/Δθ holograms in number can be multiplexed.
In a hologram recording material such as a photopolymer, a performance index called M number (hereinafter, called as “M/#”) is used. M/# represents an amount which is proportional to a square root of a diffraction efficiency and is proportional to a change in refractive index. The diffraction efficiency is a value in terms of a ratio of a diffraction beam to an incident beam, in place of percentage, and is a dimensionless number. As M/# is increased, the total diffraction efficiency is increased. A value obtained by dividing M/# with a square root of a minimal diffraction efficiency required in reproducing is the multiplicity number usable as a performance of the hologram recording material.
In the following, an idea on mechanical constraints or multiplicity limitation by a recording system in a hologram recording/reproducing apparatus is described. FIG. 15 is a schematic diagram for describing a general angular multiplexing recording system. FIG. 15 shows an arrangement relation between a cross section in thickness direction of a hologram recording medium, and a signal beam and a reference beam. In the example of FIG. 15, a signal beam 204 has a fixed incident angle with respect to a hologram recording medium 200, whereas a first reference beam 201, a second reference beam 202, and a third reference beam 203 are irradiated onto the hologram recording medium 200 with incident angles different from each other.
In the above arrangement, for instance, a hologram recorded by the signal beam 204 and the first reference beam 201 can be reproduced by causing a readout beam to be incident onto the hologram recording medium 200 with substantially the same angle arrangement as the first reference beam 201. The expression “substantially the same angle arrangement” includes a condition that the incident angle of the readout beam may not completely coincide with the incident angle of the first reference beam 201 to be used in recording due to shrinkage of the recording medium, thermal influence involved in recording/reproducing, or a like factor. Holograms are multiplex-recorded in a substantially common area of the hologram recording medium 200 by setting the incident angles of the first reference beam 201, the second reference beam 202, and the third reference beam 203 to such values capable of sufficiently separating the holograms to be reproduced by the corresponding readout beam one from the other, considering the above factors.
Hereinafter, the angle range capable of separating the holograms one from the other is called as “angle selectivity”. If, for instance, the angle selectivity is 0.01°, and the angle range of a reference beam is maximally settable to 60° with respect to the normal of the recording medium, considering the mechanical constraints, the maximum multiplicity number is 3,000. The aforementioned description leads to a conclusion that a possible multiplicity number i.e. a recording capacity can be determined substantially by a multiplicity limit depending on the aforementioned recording material, which is determined by M/#; or a multiplicity limit depending on an angle resolution performance or a wavelength resolution performance depending on a multiplex recording system, or a phase code number, or a like parameter.
Various materials such as organic materials including a photopolymer, and inorganic materials called photo refractive crystals are proposed as the hologram recording medium. A variety of researches and developments have been made in various aspects including production method and production cost, based on basic properties of the materials such as recording sensitivity, recording capacity, and information retainability. The hologram recording/reproducing technique is generally directed to an information recording/reproducing system using a light behavior i.e. photon mode. In this context, the hologram recording medium is a photosensitive member in the aspect of recording system, and has a sensitivity to light of a wavelength equal to smaller than a wavelength in a visible light wavelength band.
Hologram recording using a general holographic material e.g. a photopolymer material, is performed, by utilizing a difference in refractive index between a polymer obtained by polymerizing monomers by light irradiation, and a matrix material or a binder. In using these materials as the recording material, a reaction suppressant for inhibiting photopolymerization from initiating by a meager amount of unwanted light e.g. stray light, or a pigment for increasing the sensitivity to light of a wavelength for recording may be added.
In practice, for instance, in the case where a radically polymerizable photopolymer is used, after a light energy carrying interference pattern information is irradiated onto a recording medium, a series of processes comprising light absorption by the pigment, radical generation, polymerization, and diffusion/fixation are carried out. It is known that a certain time is required for completing an interference pattern formation (hereinafter, also called as “hologram formation”) by the series of chemical/structural changes.
In the following, a process of hologram formation is described. FIGS. 16A through 16C are conceptual diagrams for describing the hologram formation process. FIG. 16A is a diagram showing a step of irradiating light onto a hologram recording medium. FIG. 16B is a diagram showing a step of polymerizing monomers by light irradiation. FIG. 16C is a diagram showing a step of diffusively migrating monomers and a binder, which conceivably corresponds to a phenomenon of alleviating a relative monomer concentration distribution expressed in the recording medium in FIG. 16B.
For instance, as shown in FIG. 16A, light 105 is irradiated onto a hologram recording medium 101 containing monomers 102 and a binder 103 in a mixed state. The refractive index “n” of the monomers 102 is 1.45, and the refractive index “n” of the binder 103 is 1.58. In using the photopolymerizable hologram recording medium 101, the polymer 104 polymerized by the light irradiation in FIG. 16B has a relatively large refractive index “n” to the other part of the recording medium 101 i.e. the area where the binder 103 and the monomers 102 are co-existent. The refractive index “n” of the polymer 104 is 1.48.
A concentration distribution concerning the monomers 102 and the binder 103 is generated resulting from the polymer 104 formed by the polymerization. As a result, referring to FIG. 16C, the monomers 102 and the binder 103 diffusively migrate in such a manner as to alleviate the concentration distribution. Finally, a refractive index distribution shows such a characteristic that the refractive index “n” of the polymerized part formed by light irradiation is smaller than that of the peripheral part which has not undergone the light irradiation. In use of the conventional hologram recording material, a time required for interference pattern formation cannot be desirably reduced, and a long time is required to form an interference pattern, as compared with an irradiation time of a reference beam for recording, and a signal beam.
Another hologram recording material such as an organic photorefractive material or a ferroelectric liquid crystal material has also been researched and developed. Even with use of these materials, in most of the cases, an interference pattern formation time is long, as compared with a light irradiation time, because an interference pattern is formed, in other words, hologram recording is performed by utilizing a chemical and/or structural change resulting from irradiation of a reference beam and a signal beam. Also, the aforementioned interference pattern formation may progress after the irradiation of the signal beam and the reference beam for hologram recording is terminated, depending on the components constituting the recording material to be used. This is generally called a dark response.
The interference pattern formation time, irrespective of whether a dark response is included or not, or irrespective of whether the response is large or small, is determined based on a composition ratio of the ingredients constituting the recording material or a like factor, considering the basic performance of a targeted recording medium to be produced. In view of this, although it is possible to shorten the interference pattern formation time by a material design process of increasing the material sensitivity or the diffusion rate, or a like measure, it is necessary to consider other features of multiplex-recordable recording medium available as a large-capacity storage e.g. suppressing occurrence of shrinkage, or securing controllability in diffraction efficiency of each hologram.
As a general finding, there is known a performance index called recording sensitivity. The recording sensitivity is expressed by a value of a square root of a diffraction efficiency with respect to an energy amount (unit: mJ/cm2, for instance) required for recording a hologram to be irradiated per unit area. The energy amount is hereinafter called as “recording energy amount”. The diffraction efficiency is a value in terms of a ratio of a diffraction beam to an incident beam, in place of percentage, and is a dimensionless number. The recording sensitivity is assumed to have a relevancy to the amount of residual monomers, i.e. monomers which are not polymerized, in use of e.g. a photopolymerizable photopolymer. In other words, since a recording medium in an unrecorded condition has a high recording sensitivity due to the abundant existence of residual monomers, an intended diffraction efficiency can be obtained with a small irradiation energy amount.
However, as the multiplex recording progresses, the residual monomer amount is decreased, and accordingly, the recording sensitivity is degraded. To compensate for the drawback, there is proposed a method for securing a diffraction efficiency substantially equivalent to a condition before the multiplex recording progresses by increasing the irradiation energy amount. The method is e.g. disclosed in Japanese Unexamined Patent Publication No. 2005-327393 (D1) and Japanese Unexamined Patent Publication No. 2005-189748 (D2).
The above discussion also leads to an idea that the recording sensitivity is determined by integration of a given irradiation energy amount, because the recording sensitivity is affected by a monomer consumption rate. In view of this, it is possible to calculate a current recording sensitivity in a targeted area of a hologram recording medium by grasping the irradiation energy amount applied to the targeted area.
Concerning a hologram recording technique, particularly, a hologram recording technique based on multiplex recording, there is known an approach of performing scheduling concerning a recording operation (hereinafter, called as “scheduling recording”). This is an approach of setting individual diffraction efficiencies of multiplex-recorded holograms to a fixed value by controlling the recording energy amount depending on the number of times of multiplex recording, in the case where the recording sensitivity is changed with respect to a hologram recording medium.
For instance, D1 proposes, in a shift multiplexing recording system, a method for irradiating recording beam, which is the sum of a signal beam and a reference beam, by performing a multiplicity specifying step of specifying the multiplicity number of holograms recorded in a recording layer, and an irradiation condition determining step of determining an irradiation energy amount of a recording beam. Specifically, D1 discloses a method for controlling the irradiation energy amount by changing a recording irradiation time or a recording optical power.
D2 proposes a method for increasing a recording optical power depending on lowering of a recording sensitivity of a hologram recording medium by setting a recording time per data page in a multiplex recording step i.e. a light irradiation time onto the hologram recording medium to a fixed value. D2 also proposes a method for decreasing the information amount per data page depending on lowering of the recording sensitivity of the hologram recording medium by setting the recording time and the recording optical power to a fixed value. Both of D1 and D2 have an object to suppress or prevent increase of a recording time resulting from lowering of recording sensitivity of the hologram recording medium, in other words, lowering or fluctuation of data transfer rate.
A research has been made concerning hologram multiplex recording/reproducing with respect to a photopolymer, which is a material constituting a general hologram recording medium, with use of the conventional multiplex recording system including the above proposed methods. As a result of the research, there is found a phenomenon that the dynamic range is lowered i.e. recording is disabled before a current recording capacity reaches a potential recording capacity estimated to be inherently provided in the hologram recording medium, or presented by the manufacturer of the recording medium. In view of this, it is necessary to prevent lowering of the dynamic range in order to realize a high-density and large-capacity recording, which is a primary feature of the hologram recording/reproducing.